This invention relates to ceramic nuclear fuel pellets.
The service life of a nuclear reactor fuel element is dependent upon interactions between the fuel element sheath and nuclear fuel within it. These interactions may be chemical -- as in the corrosion of zirconium alloy sheaths by moisture in the fuel -- or mechanical -- by fuel swelling or fuel densification.
Sintered uranium dioxide fuel pellets in use as nuclear fuel undergo dimensional changes as a consequence of both swelling and densification. These dimensional changes have been shown to have undesirable effects on the integrity of fuel element sheaths and hence on fuel element performance. Accordingly efforts have been made to reduce dimensional instabilities resulting from the two main mechanisms which exist; these are
1. Pellet densification due to the removal of residual fine sintering porosity by irradiation and thermal processes PA0 2. Pellet swelling due to the accumulation of solid and gaseous fission products as inclusions and bubbles.
Such efforts to produce dimensionably stable uranium dioxide pellets have previously been directed to stabilising the pores in the pellets (so that the pores are not removed by prolonged sintering) by balancing the pressure of gas contained in the pores against the surface tension forces which would otherwise lead to diminution of the pores. However these pellets, manufactured by pressing and sintering ceramic uranium dioxide powder, are generally of high density (typically greater than 95% theoretical density or 10.40 g/cm.sup.3) whereas fuel specifications often require up to 10% voidage to minimise swelling due to the accommodation of fission products. For this latter purpose the uranium dioxide pellets are sintered to a density less than the thermal equilibrium value. This procedure produces a fuel which on further thermal treatment will densify to approach the equilibrium thereby causing shrinkage. Furthermore under irradiation the shrinkage may extend beyond the thermal equilibrium value. This is because in a sintered uranium dioxide pellet at equilibrium most of the pore volume is contained in pores less than a few microns in diameter, which (in order to be stable) must contain gas at pressures of over 10 atmospheres, and it has been shown that these small pores are readily removed during irradiation. Thus a pellet densifying in reactor conditions will not, in general, merely reach the thermal equilibrium value but will continue to approach the theoretical density unless prevented by other mechanisms, for example, swelling induced by fission products. It therefore follows that the porosity incorporated in a conventional uranium dioxide fuel pellet is unstable and dimensional changes result during reactor operation. The present invention seeks to provide fuel pellets of uranium dioxide in which dimensional instability is reduced.